Totally white shadeless material in Cycles?
Here is a node setup for a shadeless material:


If you use just an emission shader the object produces light.  So it still is not shadeless as it is casting light on other objects. To make it shadeless, you must make sure it sends light only to the camera. To do this, add mix shader with the emission shader plugged in the bottom node socket. Leave the mix shader's top node socket empty (or plug in a holdout shader if the empty socket bothers you). Then plug the is camera ray from a light path input node into the factor of the mix shader.